Communication systems designed to incorporate the characteristic of communicating with many remote mobile stations for brief intervals and occupying common radio spectrum resources are termed multiple access communication systems. One type of communication system which can be a multiple access system is a spread spectrum system. In a spread spectrum system, a modulation technique is utilized in which a transmitted signal is spread over a wide frequency band within the communication channel. The frequency band is much wider than the minimum bandwidth required to transmit the information being sent. A voice signal, for example, can be sent with amplitude modulation (AM) in a bandwidth only twice that of the information itself. Other forms of modulation, such as low deviation frequency modulation (FM) or single sideband AM, also permit information to be transmitted in a bandwidth comparable to the bandwidth of the information itself. However, in a spread spectrum system, the modulation of a signal to be transmitted often includes taking a baseband signal (e.g., a voice channel) with a bandwidth of only a few kilohertz, and distributing the signal to be transmitted over a frequency band that may be many megahertz wide. This is accomplished by modulating the signal to be transmitted with the information to be sent and with a wideband encoding signal.
Information (i.e. a message signal) can be embedded in the spread spectrum signal by several methods. One method is to add the information to the spreading code before it is used for spreading modulation. This technique can be used in direct sequence and frequency hopping systems. It will be noted that the information being sent must be in a digital form prior to adding it to the spreading code, because the combination of the spreading code and the information, typically a binary code, involves modulo-2 addition. Alternatively, the information or message signal may be used to modulate a carrier before spreading it.
Thus, a spread spectrum system must have two properties: (1) the transmitted bandwidth should be much greater than the bandwidth or rate of the information being sent and (2) some function other than the information being sent is employed to determine the resulting modulated channel bandwidth.
A specific type of spread spectrum communication technique known as direct sequence works by modulating a carrier with a digital code sequence whose bit rate is much higher than the information signal bandwidth.
Multiple communication channels are allocated by using a plurality of spreading codes within a portion of radio spectrum, each code being uniquely assigned to a mobile station. The unique spreading codes are preferably orthogonal to one another such that the cross-correlation between the spreading codes is approximately zero. Particular transmitted signals can be retrieved from the communication channel by de-spreading a signal representative of the sum of signals in the communication channel with a spreading code related to the particular transmitted signal which is to be retrieved from the communication channel. Further, when the spreading codes are orthogonal to one another, the received signal can be correlated with a particular spreading code such that only the desired signal related to the particular spreading code is retrieved.
Handoff of the spread spectrum mobile between cell-sites in a spread spectrum system is accomplished by using the Mobile Assisted Handoff (MAHO) functions of the mobile stations. Typically, the spread spectrum mobile station makes measurements on pilot channels belonging to each cell-site. Pilot channels are unique codes in the spread spectrum system that the mobile station knows of a priori. The measurement of any one pilot channel is composed of a strength indication and a phase measurement of the pilot relative to a time standard used by the spread spectrum system. The list of pilot channels for the mobile station to use in determining handoff suitability is communicated to the mobile station at call setup time or via a special message during traffic channel operations. When the mobile station observes that the measured strength of a particular pilot channel exceeds a predetermined threshold, the mobile station shall signal the base station with this information in a request for handoff. Handoff is then enacted at the discretion of the base station equipment.
A special form of handoff known as "soft handoff" is possible in the direct sequence spread spectrum system. This is due to the fact that all communications between mobile stations and base stations take place on the same frequency/communications channel. Soft handoff allows the mobile station to communicate with several base stations simultaneously, thus enhancing reception by both the base station equipment and mobile stations.
Narrowband frequency modulation (FM) systems impart intelligence to a carrier by varying the frequency of the carrier within a predetermined range (maximum deviation). Such systems are known, and one such example applied to cellular communications is the Advanced Mobile Phone Service (AMPS) cellular system specified by the Electronic Industries Association. Because narrowband systems like AMPS do not support MAHO, handoff of a mobile station from one cell-site to another cell-site requires the intervention of the cellular system. The Mobile Switching Center (MSC) in a narrowband system determines possible target candidates for a call requiring handoff. Specifically, a scanning, or locating, receiver may be used at the current serving cell-site to detect the need for handoff (based on low received signal strength) and at the target candidate cell-site (by command of the MSC) to determine the suitability of that cell to support the call. While many handoff algorithms could, and do, exist, a handoff is typically enacted when one cell-site perceives the mobile station's signal strength to be stronger than the current serving cell-site.
As spread spectrum technology becomes incorporated into next generation cellular systems, practical system complications due to the nature of cellular systems arise. For example, in cellular systems incorporating both spread spectrum and narrowband FM channels it may become necessary to enact handoff between both sets of channels when conditions warrant. Such a handoff could only take place for mobile stations that are dual mode in nature such that they support both spread spectrum and narrowband signalling. While a mobile station is communicating with the cellular system on a spread spectrum channel, it is not possible for the narrowband portion of the system to determine handoff suitability of the mobile station as the transmissions are spread over a bandwidth larger than the narrowband channel. In addition, the FM portion of the cellular system does not support spread spectrum pilot channels. Systems may be constructed such that the spread spectrum portion of the system and the narrowband portion of the system are non-overlapping. Thus, a cellular system employing both spread spectrum coverage and narrowband coverage would need to enact handoff to preserve the continuity of a call as a mobile station moves from the area of the former to that of the latter. This is possible as mobile stations designed for the cellular system are dual-mode and are capable of generating, and interpreting, both spread spectrum and narrowband signalling. Prior art solutions to handoff from the spread spectrum portion of the system to the narrowband portion of the system assumed overlap of RF coverage between the portions. Thus, there existed no direct way to guarantee that the mobile station would experience adequate RF coverage upon completion of the handoff in a non-overlapping system.
Thus a need exists for a mechanism by which handoff may take place from the spread spectrum portion of a cellular system to the narrowband portion without increasing the equipment cost of the narrowband equipment suite while guaranteeing that a mobile station has adequate RF coverage within the narrowband portion of the cellular system upon completion of the handoff process.